1. Technical Field
The present invention relates to a semiconductor device comprising a semiconductor chip, and relates, in particular, to a semiconductor device having an insulated gate field effect transistor mounted thereon.
2. Related Art
An exemplary conventional technology related to a semiconductor device having a power MOSFET mounted thereon is disclosed in Japanese Patent Laid-Open No. 2001-274,206 (pp. 3 to pp. 4, FIG. 1 and FIG. 2). FIG. 8 is a plan view of a power MOSFET mounted on a semiconductor package described in Japanese Patent Laid-Open No. 2001-274,206, FIG. 9 is a cross-sectional view thereof along line A-A and FIG. 10 is an enlarged cross-sectional view of a portion thereof between electrodes. A semiconductor chip 101 of the power MOSFET is mounted on an island 103 of a lead frame 102, and an upper surface of the semiconductor chip 101 is provided with a source electrode 104 and a gate electrode 105 of a transistor formed thereon, which are composed of aluminum or aluminum alloy. The lower surface of the semiconductor chip 101 is provided with a drain terminal (not shown), which is coupled to the island 103. Clip-shaped metallic plates 108 and 109 composed of copper belt or copper alloy foil are employed as coupling conductors for electrically coupling the source electrode 104 and the gate electrode 105 to a source lead 106 and a gate lead 107, respectively, and the source electrode 104 and the gate electrode 105 are joined to the clip-shaped metallic plates 108 and 109, respectively, through electrically conducting pastes 110. In addition, the other ends of the clip-shaped metallic plates 108 and 109 are also electrically coupled to the source lead 106 and the gate lead 107, respectively, through the electrically conducting paste 110. Then, the semiconductor chip 101 is encapsulated with a resin 112, together with the island 103, the source lead 106, the gate lead 107 and the drain lead 111. Normally, the source electrode 104 is electrically isolated from the gate electrode 105 by an insulating cover film 113 composed of silicon nitride, phosphorus glass and the like. Only a certain range of materials limited to a certain extent may be selected for the metal composing the outermost surfaces of the source electrode 104 and the gate electrode 105, in order to provide a better coupling to an external terminal. In particular, electrode materials such as Ag, Cu, Pb—Sn and the like are selected, when a coupling with an electrically conducting paste material or a junction employing solder is provided in place of providing an ordinary coupling with a bonding line of gold or aluminum.
However, since the electrode material such as Ag, Cu, Pb—Sn and the like is a material that is more likely to cause so-called metallic ion migration as compared with Al, in which metallic ion moves toward a region having lower electric potential when there is a gradient in the electric potential at higher temperature in a highly humid condition, as disclosed in Japanese Patent Laid-Open No. H09-148,520 (1997) (pp. 2 to pp. 3), a material composing an electrode in the side of higher potential that is located in the side of the anode is ionized and moved therefrom, and then is precipitated and grown up at an electrode in the side of lower potential that is located in the side of the cathode, when a current test is conducted at higher temperature in a highly humid condition under the status of applying an electrical voltage between the electrodes, thereby providing a condition of easily causing defective situations that may lead to an electrical leakage and/or a short circuit.